Cotton has been cultivated for its fiber for over 7000 years. Despite the availability of synthetic alternatives, it continues to serve as the most important source of fiber for textiles. Cotton is grown in more than eighty countries and is a cash crop for more than 20 million farmers in developing countries in Asia and Africa where malnutrition and starvation are rampant (De Onis et al., 1993). An attribute of cotton not widely recognized is that for every 1 kilogram (kg) of fiber, the plant produces approximately 1.65 kg of seed. This makes cotton the third largest field crop in terms of edible oilseed tonnage in the world. However, the ability to utilize the seed and oil is hampered by the presence of a toxic terpenoid, gossypol. The presence of gossypol, a cardio- and hepatotoxic terpenoid unique to the tribe Gossypieae, in the seed glands renders cottonseed unsafe for human and monogastric animal consumption (Risco & Chase Jr., 1997).
A major portion of this abundant agricultural resource is utilized as feed for ruminant animals either as whole seeds or as meal following oil extraction; however, if consumed in sufficient amounts, cottonseed diminishes the reproductive performance of bulls (Chenoweth et al., 1994). During the processing of cottonseed, which involves moist heating, a double bond is formed between the ε-amino group of lysine and the aldehyde group in gossypol. Although bound gossypol is less toxic, the bioavailability of soluble protein and lysine in the meal is reduced. Additional chemical processing steps are needed to remove gossypol to make the oil fit for human use. Consumption of improperly refined oil has been known to cause sterility in men.
Cottonseed also contains 22.5% protein by weight of relatively high quality. The 44 million metric tons (MT) of cottonseed produced each year could provide the total protein requirements of half a billion people per year (50 g/day rate−9.4 million MT of available protein) if the seed were safe for human consumption. However, it is woefully underutilized because of the presence of toxic gossypol within seed glands (Lusas & Jividen, 1987). Thus, gossypol-free cottonseed represents an enormous source of protein that could significantly contribute to human nutrition and health particularly in developing countries (Bressani, 1965; Lambou et al., 1966; Alford et al., 1996) and help society meet the requirements of the predicted 50% increase in the world population in the next 50 years.
Gossypol and related terpenoids are also present throughout the cotton plant in the glands of bolls and foliage, and in roots. In addition, these terpenoids are also induced in response to microbial infections. These compounds protect the plant from both insects and pathogens (Hedin et al., 1992; Stipanovic et al., 1999). Elimination of gossypol from cottonseed has been a long-standing goal of geneticists. Attempts were made in the 1950s to meet this objective by developing so-called “glandless cotton” via conventional breeding techniques (McMichael, 1954; McMichael, 1959; McMichael, 1960; Miravalle & Hyer, 1962; Lusas & Jividen, 1987). Following the discovery of a glandless mutant (McMichael, 1954), several breeding programs were launched in the United States of America, Africa, and Asia to transfer the glandless trait into commercial varieties to produce gossypol-free cottonseed (McMichael, 1959; McMichael, 1960; Miravalle & Hyer, 1962; Lusas & Jividen, 1987). These programs provided cottonseed that could be fed to the more efficient feed-utilizing, monogastric animals and was even deemed safe for human consumption. However, these cotton varieties were a commercial disaster. Under field conditions, glandless plants were extraordinarily susceptible to attack by a host of insect pests because they constitutively lacked protective terpenoids (Bottger et al., 1964; Jenkins et al., 1966) and therefore, were rejected by the farmers.
During the last decade, a number of attempts have been made to utilize antisense technology to eliminate gossypol from cottonseed. However, these attempts have either been unsuccessful (Townsend et al., 2005), have resulted in a small reduction in seed gossypol, or have provided ambiguous results (Martin et al., 2003; Benedict et al., 2004). Despite the advances that have been made toward eliminating gossypol from cottonseed, the production of sufficiently useful products has remained elusive and the potential of cottonseed in contributing to human nutrition has remained unfulfilled.